1. Fields of the Invention
This invention relates to a combustion apparatus for reducing the amount of nitrogen oxides (hereinafter abbreviated to NO.sub.x) and particularly it relates to a combustion apparatus capable of achieving a very low content of NO.sub.x at the time of burning pulverized coal.
2. Description of the Prior Art
Due to change in the recent fuel situation, large-scale boilers for such establishments as thermal power stations wherein coal is used as their fuel have been increasing. In this case, coal has been pulverized into e.g. pulverized coal of which about 70% is 200 meshes pass to improve combustibility and controllability.
As is well known, however, NO.sub.x byproduced by combustion is likely to be generated in high-load combustion burners and it has been one of main causes of air pollution; thus, certain basic improvements in burners or improvement in combustion over the whole of combustion furnaces have been made. A particular problem raised in the combustion of pulverized coal is that an organic-type nitrogen (hereinafter referred to as Fuel N) contained in a large amounts (usually 1 to 2% by weight) in pulverized coal generates NO.sub.x and this NO.sub.x constitutes most part of the NO.sub.x generated in combustion.
Now, the respective formation reactions of NO.sub.x and N.sub.2 from the Fuel N are expressed by the following equations (1) and (2) and these two reactions are competitively carried out: ##STR1##
Thus, in order to make N.sub.2 formation predominant and maintain a high-load combustion, it is important to ensure a high temperature reducing flame.
In general, a combustion process referred to as two-stage combustion is an application of this combustion reaction. Namely, as shown in FIG. 1, an air-deficient zone is formed in the burner zone 53 of a combustion furnace 51 and an amount of air corresponding to the above deficient amount of air is supplied through the so-called after air port 57 provided downstream of burners 55 to effect complete combustion, whereby combustion over the whole of the combustion furnace is improved to thereby reduce the amount of NO.sub.x discharged. In the case of newly established boilers using general coals as fuel, the concentration of NO.sub.x discharged therefrom has currently come to be reduced down to about 200 ppm.
However, in the case of the two-stage combustion, half-burned coal particles (char) are formed in the air-deficient burner zone, and it requires a large free space in the furnace for complete combustion of the char with after-air. Thus, although the above combustion process is very effective in lowering NO.sub.x in the combustion, it has still a certain limitation.
Thus, the so-called dual resistor type burner has been developed which is constructed so that the respective burners can effect a low NO.sub.x combustion based on the above principle, in place of controlling the combustion over the whole of boilers. FIG. 2 illustrates the dual resistor type burner. Pulverized coal is carried by a carrier air (primary air) in an amount of about 20 to 30% of combustion air, passed through a pulverized coal pipe 8 in the form of pulverized coal stream, and injected through an injection port 9 into a combustion furnace. This pulverized coal stream is burned within the combustion furnace in a low air ratio, to form reducing intermediate products and reduce a part of NO.sub.x in gas phase. On the other hand, at the outer peripheral part of the flame formed by combustion of the pulverized coal stream is fed through an injection port 11, a secondary air 4 passed through a secondary air resistor 12 and having a whirling force imparted by an air vane 16, and further at the outer peripheral part thereof is fed through an injection port 7, a ternary air 6 passed through a ternary air resistor 14. Thus, air is fed to the flame after the gas phase reduction to burn unburned matters. In such a manner, a two-stage combustion is carried out by means of a single burner, and reduction of NO.sub.x down to about 400 ppm (percentage reduction : about 40%), for example, has been demonstrated. In order to achieve a low NO.sub.x concentration by means of such a type burner, it is required that the burner flame be separated from the secondary air and the ternary air in the vicinity of a burner throat 18 in the combustion furnace to form a good reducing atmosphere, and also that to the contrary, downstream of this flame, the flame (or gas) be mixed with these airs to well burn unburned matters. In the case of such a type burner, however, although the secondary air 4 is usually separated from the ternary air 6 by means of a sleeve 10, practically it has been found that the pulverized coal stream, the secondary air stream and the ternary air stream readily mix together in the vicinity of the exit of the burner throat to make it difficult to sufficiently separate and maintain the high temperature reducing flame at the initial stage of the combustion. Further, the flame-maintenance by way of conventional type burners has resorted to impellers of the so-called broad-angle spread type to make it very difficult to cause the high temperature reducing flame to exist in the vicinity of the central axis of burner in a concentrated manner.